Add a page to the documentation describing transactional durabililty.

refs #1150
whitespace
Simplify headers
Structure durability wording as 3 pages, "checkpoint durability",
"commit-level durability", and "tuning durability".
Add thread-failure wording.
document that hot backups and incremental archival can mess up with
bulk-load because it's not logged.
note there must always be at least one log file.
Wordsmith the hot backup paragraph for commit-level durability.
Merge branch 'develop' into durability-docs

Conflicts:
	src/docs/checkpoints.dox
Merge pull request #1178 from wiredtiger/durability-docs

Add a page to the documentation describing transactional durabililty.
log/log.c:477:20: error: conversion to 'off_t' from 'long unsigned int' may change the sign of the result [-Werror=sign-conversion]
Fix the size of the value returned when stepping into log records.  We skip the first byte, so we need to adjust the size.

Thanks, AddressSanitizer and valgrind!

refs #1171